Cap for a writing instrument



FIG. 1 is a perspective view of a cap for a writing instrument showing a new design;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a left view thereof;

FIG. 5 is a right view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines are included for the purpose of showing portions of the cap for a writing instrument that form no part of the claimed design. 

CLAIM The ornamental design for a “cap for a writing instrument,” as shown and described. 